This invention relates to benzodiazepine dimers having a heteroaromatic group between the two dimer units, dimer-linker compounds derived therefrom, conjugates thereof, and methods for their preparation and use.
Some naturally occurring cytotoxins, such as tomaymycin and anthramycin, contain a benzodiazepine ring system. Reflecting the additional presence of a pyrrolidine ring fused to the diazepine ring, these compounds are often referred to as pyrrolobenzo-diazepines, or PBDs.

PBDs possess antibiotic and antitumor activity, the latter trait leading to interest in them as anticancer drugs. Mechanistically, PBDs bind to the minor groove of DNA in a sequence selective manner and alkylate the DNA. The structure-activity relationship (SAR) of different substituents has been studied (Antonow et al. 2010; Thurston et al. 1999).
Additional studies have shown that PBD dimers show special promise as anticancer agents. The core structure of a typical PBD dimer can be represented by formula (A-1), where X is a bridging group connecting the two dimer halves.

As with monomeric PBDs, the dimers are DNA minor groove binder-alkylators. Being bifunctional, alkylation by a dimer results in cross-linked DNA, making DNA repair more difficult. (DNA alkylation occurs via the imine group. PDBs having one of the imine groups reduced can still alkylate DNA, but cannot crosslink it. They are still biologically active, albeit generally less so, but their different pharmacokinetic profile may be preferable for some applications.) For a review on the evolution of PBDs as antitumor agents, from naturally occurring monomers to synthetic monomers to synthetic dimers, see Hartley 2011.
The SAR of PBD dimers has been explored via substituents on the A/A′ and C/C′ rings, unsaturation in the C/C′ rings, the structure and length of the bridging group X, and the oxidation or reduction of the imine double bonds in rings B/B′, and combinations of such features. See Bose et al. 1992, Gregson et al. 1999, Gregson et al. 2001a and 2001b, Gregson et al. 2004, Gregson et al. 2009, Hartley et al. 2012, Howard et al. 2007, Howard et al. 2009a. Howard et al. 2010, Howard et al. 2013a and 2013b, Liu et al. 2007, Thurston et al. 1996, Thurston et al. 2006, and Thurston et al. 2008. Most PBD dimers are joined via an 8/8′ bridge as shown above, but a 7/7′ bridge also has been disclosed (Howard et al. 2009b).
A type of anticancer agent that is generating strong interest is an antibody-drug conjugate (ADC, also referred to as an immunoconjugate). In an ADC, a therapeutic agent (also referred to as the drug, payload, or warhead) is covalently linked to an antibody whose antigen is expressed by a cancer cell (tumor associated antigen). The antibody, by binding to the antigen, delivers the ADC to the cancer site. There, cleavage of the covalent link or degradation of the antibody leads to the release of the therapeutic agent. Conversely, while the ADC is circulating in the blood system, the therapeutic agent is held inactive because of its covalent linkage to the antibody. Thus, the therapeutic agent used in an ADC can be much more potent (i.e., cytotoxic) than ordinary chemotherapy agents because of its localized release. For a review on ADCs, see Schrama et al. 2006.
PBD dimers have been proposed as the drug in an ADC. Attachment of the linker connecting to the antibody can be via a functional group located in a C/C′ ring, the bridging group X, or by addition across the imine group in a B/B′ ring. See Beau-Larvor et al. 2014, Bouchard et al. 2013, Commercon et al. 2013a and 2013b, Flygare et al. 2013, Gauzy et al. 2012, Howard 2104a-2014e, Howard et al. 2011, Howard et al. 2013c and 2013d, Howard et al. 2014a-2014h, Jeffrey et al. 2013, Jeffrey et al. 2014a and 2014b, and Zhao et al. 2014.
Another type of benzodiazepine dimer also has been proposed as a drug for use in ADCs. Structurally, this type may be viewed as a PBD dimer further having a phenyl ring fused to each of C/C′ rings, as shown in formulae (A-2) and (A-3). See Chari et al. 2013, Li et al. 2013, Fishkin et al. 2014, Li et al. 2014.

Benzodiazepine compounds having other ring systems, such as a tetrahydro-isoquinolino[2,1-c][1,4]benzodiazepine, also have been disclosed. Kothakonda et al. 2004.
Full citations for the documents cited herein by first author or inventor and year are listed at the end of this specification.